暑さ【あつさ】
heat (of the weather), hotness
猛暑【もうしょ】
heat wave, fierce heat
残暑【ざんしょ】
late summer heat, lingering summer heat
酷暑【こくしょ】
intense heat, severe heat
避暑【ひしょ】
escaping the summer heat, going somewhere cooler during the summer, summering
暑い【あつい】
hot, warm, sultry, heated, passionate, impassioned, burning (desire, etc.), on everybody's mind, on the radar, du jour, interested (gaze, etc.)
蒸し暑い【むしあつい】
humid, sultry
暑中【しょちゅう】
height of summer, high summer, hottest part of the summer